Viscosity terms Links

Equation of Motion. A gel consists of a cross-linked network immersed in a fluid, so coupled equations should be used to describe the motion of the solvent molecules and of the polymer network. However, the motion of solvent molecules is about 10 times faster than that of the polymer network. Thus, on the time scale of the thermal fluctuations of the network, solvent effects are always averaged. An equation of motion for a gel network was proposed by Tanaka et al. (9). In the derivation of the equation of motion, solvent effects are taken into account only through a viscosity term. 

Long-term compression set resistance is described in Figure 4. Lower set values are achievable by use of higher viscosity gumstock at comparable cross-link densities. Compression set resistance is also very dependent on the cure system chosen. The bisphenol cure system offers the best compression set resistance available today, as shown in Table 5. 

Thermal Properties of Metallic Solids. In the preceding sections, we saw that thermal conductivities of gases, and to some extent liquids, could be related to viscosity and heat capacity. For a solid material such as an elemental metal, the link between thermal conductivity and viscosity loses its validity, since we do not normally think in terms of solid viscosities. The connection with heat capacity is still there, however. In fact, a theoretical description of thermal conductivity in solids is derived directly from the kinetic gas theory used to develop expressions in Section 4.2.1.2. 

In Eq. (38) the hydrodynamic velocity is that used to evaluate the momentum of the phase. In viscous flow it is the term used in establishing the shear from a knowledge of the viscosity when effects of cross linking of fluxes (01, 02) are neglected. The hydrodynamic velocity and the diffusional velocity are related by... 

The term is often called the dynamic viscosity. It is an energetic dissipation term related to G"(o>) and has a value approaching that of the steady llow viscosity t] in very low frequency measurements on polymers which are not cross-linked. 

Although all theoretical approaches discussed in previous sections do refer to a particular molecular model, rather they represent attempts to rationalize experimental kinetic data in terms of mean values of electrical field or ion distribution, energy barrier height, energy change, ion mobility and viscosity of medium, which are all supposed to be closely related to the molecular properties of the ions and solvent molecules involved. However, no direct link to molecular properties has been established within the framework of the models discussed above. 

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